update style guide to reflect NULLPTR and doxygen

cpp/README.md

@@ -322,9 +322,14 @@ This requires [Doxygen](http://www.doxygen.org) to be installed.
 
 ## Development
 
-This project follows [Google's C++ Style Guide][3] with minor exceptions. We do
-not encourage anonymous namespaces and we relax the line length restriction to
-90 characters.
+This project follows [Google's C++ Style Guide][3] with minor exceptions: 
+  
+  *  We do not encourage anonymous namespaces 
+  *  We relax the line length restriction to 90 characters.
+  *  We use the NULLPTR macro defined in `src/arrow/util/macros.h` to
+     support building C++/CLI (ARROW-1134)
+  *  We use doxygen style comments ("///") instead of line comments ("//")
+     in header files.
 
 ### Memory Pools